1. Field of Invention
The present invention pertains to the field of electronic amplifiers. More particularly, this invention relates to amplifiers having multiple gain states.
2. Art Background
It is often desirable in a variety of electronic systems to employ an amplifier that is capable of multiple gain states. For example, it is often desirable in a radio receiver to employ an amplifier having a high gain state that may be used when a received signal is relatively weak and a low gain state that may be used when a received signal is relatively strong. Such changes in received signal strength may be caused, for example, by movement of a mobile radio to different distances relative to its base station.
Some prior multiple gain state amplifiers employ an attenuator and a switch that opens and closes to add or remove the attenuator from an input signal path to the amplifier. Unfortunately, such prior amplifiers usually suffer from the excessive noise and signal loss caused by the switch. Moreover, such prior amplifiers typically consume full electrical current in both the high and low gain states which may cause excessive power consumption in a mobile device.
Other prior multiple gain state amplifiers employ a bypass switch to route an input signal directly from the input to the output of the amplifier to provide the low gain state. Typically, such a bypass switch arrangement causes a 180-degree phase shift in the transfer function of the amplifier between high and low gain states. Unfortunately, such an abrupt phase shift when it occurs in digital communications typically causes undesirable phase changes in a base band data stream. Moreover, such prior amplifier usually imposes difficulties in maintaining an impedance match at the amplifier input/output ports.
Still other prior multiple gain state amplifiers employ mechanisms for switching the gain of the amplifier by switching different values of series or shunt feedback resistance, bias voltages, etc. Unfortunately, such techniques typically have an undesirable impact on input and output port impedance match.
Other prior multiple gain state amplifiers employ multiple amplifiers and switches for switching an input signal path among the amplifiers. Unfortunately, such amplifiers typically suffer from the excessive noise and signal loss caused by the switches. In addition, such techniques typically increase the hardware costs due to the presence of the additional amplifiers and related components.